1. Field of the Invention
This invention relates to a current sensing apparatus, and more particularly, to a current sensing apparatus that does not require any ohmic connection between the sensing apparatus and the feeder cable carrying the current.
The term "ohmic connection" as used herein refers to a direct wire connection, such as by a metal contact probe or clamp as employed in conventional measurement instruments.
2. Description of the Relevant Art
Many devices are available for measuring AC electrical current flowing in an electrically conductive wire. Generally, these instruments are of a split-core current transformer type in which the transformer core is opened and then clamped around an insulated single conductor to measure the current flowing in it. However, portable instruments operable without an ohmic connection have been limited to making current measurements on a single wire (line) since, if the open transformer core encompasses a conductor pair the magnetic field passing through the core will be canceled yielding either no output or an erroneous output.
The most pertinent reference of which we are aware is U.S. Pat. No. 3,056,922 issued to W.E. DuVall, et al on Oct. 2, 1962. The apparatus disclosed therein includes a coil having a magnetic core extending therethrough. A pair of pole pieces are connected to opposite ends of the core and extend, essentially in the form of an L, from the core to the pole tips, which are opposed to one another, and define therebetween an air gap. This air gap is made substantially smaller than one of the conductors in the cable in which the measurement is to be made. Preferably, the current sensor is made sufficiently small to be substantially shielded by one of the conductors from the magnetic field of the other conductors. Means are provided to attach the pick-up head to the cable in which the current is being measured with the air gap extending parallel to one of the conductors. Preferably, the sensing head is positioned with its air gap on a line which passes through the centers of the conductors in the cable under measurement. However, the apparatus disclosed therein was not meant to accurately measure the current flowing in the cable, but rather to provide an induced output voltage which would indicate that current had started to flow and no attempt was made to obtain linearity (ratio constant) between the current flowing in the conductor and the induced voltage in the pick-up coil. The air gap between the pole pieces was made much smaller than the cable to be measured and consequently the magnetic field passing through the core, via the pole pieces, included only the field generated by the current flowing in one of the conductors and made no use of the field formed by the current flowing in the other conductor.
In order to overcome the limitations of the known art and provide a means for providing a directly proportional and constant ratio between induced coil voltage and current flowing in a cable pair the apparatus of the instant invention was conceived.
Therefore, it is an object of the present invention to provide a current sensing apparatus that is capable of measuring current flowing in a pair of wires of a power feeder cable.
It is yet another object of the present invention to provide a current sensing apparatus suitable for portable field use easily attachable to a feeder cable and removable therefrom without requiring ohmic contact.
It is yet another object of the present invention to provide a current sensing apparatus that is accurate, linear, and provides a constant ratio of induced output voltage for current flowing in a cable pair.
It is still yet another object of the present invention to provide a current sensing apparatus that utilizes the magnetic fields generated by current flowing in a pair of wires of a feeder cable to provide an induced output voltage proportioned to the current flowing.
It is still yet another object of the present invention to provide a current sensing apparatus that can accurately measure current flowing in a pair of conductors, has an extended current range and is suitable for use with a plurality of feeder cables just by changing the proportionality (ratio) constant.
It is yet still another object of the present invention to provide an inexpensive current sensing apparatus which may be utilized together with other instruments to provide an accurate power rating for equipment in use.
It is yet further another object of the present invention to provide a sensing apparatus capable of sensing current flowing in a pair of wires in a power feeder cable and provide an output voltage proportionate to the amount of current flowing in the feeder cable.